Method of fabricating semiconductor devices employing at least one modulation doped quantum well structure and one or more etch stop layers for accurate contact formation

ABSTRACT

A method of fabricating a semiconductor device includes the steps of forming (or providing) a series of layers formed on a substrate, the layers including a first plurality of layers including an n-type ohmic contact layer, a p-type modulation doped quantum well structure, an n-type modulation doped quantum well structure, and a fourth plurality of layers including a p-type ohmic contact layer. Etch stop layers are used during etching operations when forming contacts to the n-type ohmic contact layer and contacts to the n-type modulation doped quantum well. Preferably, each such etch stop layer is made sufficiently thin to permit current tunneling therethrough during operation of optoelectronic/electronic devices realized from this structure (including heterojunction thyristor devices, n-channel HFET devices, p-channel HFET devices, p-type quantum-well-base bipolar transistor devices, and n-type quantum-well-base bipolar transistor devices). The etch stop layer(s) preferably comprise AlAs that functions as an etch stop during etching by a chlorine-based gas mixture that includes fluorine. The series of layers preferably comprise group III-V materials.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.10/280,892, filed Oct. 25, 2002, entitled “Optoelectronic DeviceEmploying At Least One Semiconductor Heterojunction Thyristor ForProducing Variable Electrical/Optical Delay,” now U.S. Pat. No.6,954,473 commonly assigned to assignee of the present invention, andherein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to field of semiconductor devices (andassociated fabrication methodology) and, in particular, to semiconductordevices (and associated fabrication methodology) that utilize modulationdoped quantum well heterojunctions to realize optoelectronic/electronicdevices.

2. State of the Art

Modulation-doped quantum well heterojunction transistors—including wellknown Pseudomorphic Pulsed Doped High Electron Mobility Transistors(Pulsed Doped PHEMT), which are sometimes referred to as Pulsed DopedModulation Doped Field Effect Transistors (Pulsed Doped MODFET) orPulsed Doped Two Dimensional Gas Field Effect Transistors (Pulsed DopedTEGFET)—have become well recognized for their superior low noise andhigh frequency performance and are now in demand in many high frequencyapplications (e.g., front end amplifier in wireless communicationssystems and in Monolithic Microwave and Millimeterwave IC (MMIC)designs).

GaAs/InGaAs/AlxGa_(1-x)As is the III-V material system of choice forthese devices because of the ability to grow high optical/electricalquality epitaxial layers by molecular beam epitaxy (MBE). Alternatively,strained silicon heterostructures employing silicon-germanium (SiGe)layers have been used to produce such devices.

U.S. Pat. No. 4,827,320 to Morkoc et al. discloses a pseudomorphic HEMT(PHEMT) structure that employs a layer of strained InGaAs (undoped)between a GaAs substrate and a layer of undoped AlGaAs to form a quantumwell (QW) defined by the strained InGaAs layer. A layer of n+ dopedAlGaAs is formed on the undoped AlGaAs layer. A layer of n+ GaAs isformed on the layer of n+ doped AlGaAs. The layer of n+ GaAs facilitatesan ohmic contact to source/drain electrodes. A gate electrode ofaluminum is recessed below the layer of n+ GaAs and a portion of the n+AlGaAs layer by wet chemical etch and evaporation of aluminum.

The PHEMT structure has been very successful in producing microwavetransistors that operate well into the multi-gigahertz regime, initiallybeing used extensively in military systems and now finding their wayinto commercial products, particularly in the area of cellularcommunications. In recent years, there has been a growing interest incombining the PHEMT with optical capability because of the difficulty inpropagating very high frequency signals to and from the integratedcircuit by coaxial lines. Combining electronic with optoelectroniccomponents monolithically gives rise to the concept of theoptoelectronic integrated circuit (OEIC). However, there are seriousproblems encountered because of the dissimilar nature of the structuresof the FET, the pn junction laser, PIN diode, etc.

To achieve this goal, inversion channel heterojunction structurescreated from a single epitaxial growth have been used to realize a rangeof optoelectronic devices including lasers, detectors and field effecttransistors (FETs). An exemplary inversion channel heterojunctionstructure is described in Taylor and Kiely, “Theoretical andExperimental Results for the Inversion Channel Heterostructure FieldEffect Transistors”, IEE Proceedings-G, Vol. 140, No. 6, December 1993.In this structure, for the region between the modulation doping layerand the gate of the semiconductor surface, the doping of this region issubstantially p type in order to provide a low resistance ohmic contactfor the gate of the FET.

However, the high p-type doping of this region creates many problems,including:

-   -   i) the effects of free carrier absorption makes formation of a        vertical cavity laser difficult;    -   ii) forming a depletion-type FET by implanting n-type dopant is        difficult; this difficulty stems from the difficulty in        controlling the dopant density in the bulk region; more        specifically, compensating a large p density with a large n        density to obtain a lower p density is difficult to control in a        bulk region (but much easier in a delta doped region);    -   iii) controlling the threshold voltage of an enhancement type        FET is difficult because the input capacitance is a function of        doping which is harder to control than layer thickness; and    -   iv) producing effective current funneling for inducing lasing is        difficult; more specifically, it is very desirable to create a        pn junction by N type implantation to steer the current in this        structure since this would be compatible with the overall        approach to building the FET devices; the heavy p doping bulk        layers makes it difficult to create junction isolation that has        low leakage.

Heterojunction Bipolar Transistor (HBT) devices have also been developedfor high frequency applications. An HBT device includes a base layerstructure disposed between an emitter layer structure and a collectorlayer structure. The base layer structure may utilize a gradedcomposition (as described in U.S. Pat. No. 6,037,616) or a modulationdoped QW structure (as described in U.S. Pat. No. 5,003,366). Atransferred-substrate process may be used wherein the emitter isepitaxially grown on a substrate, and the collector is epitaxially grownon the top of the sample. By depositing the collector as a small featureon the top surface of the sample and etching a collector mesa, a minimumcollector capacitance is realized. At this point, the sample is flippedand mounted on a low resistance ground plane, and the substrate belowthe emitter is removed by etching so that processing of the emitter andbase can begin in a conventional manner from the top side. An exemplarytransferred-substrate process for HBTs is described in D. Mensa et al.,“Transferred-substrate HBTs with 254 GHz F_(T),” Electron. Lett., April1999, 35(7), pp. 605-606. These prior art devices provide for improvedcurrent gain and cutoff frequency with respect to prior art siliconbipolar transistors. However, it is difficult to realize a range ofoptoelectronic devices (including lasers, detectors, FET devices,waveguide devices) from the epitaxial growth that is used to form suchHBT devices.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a single layerstructure which can be used to realize within a single integratedcircuit chip a wide range of optoelectronic devices (including lasers,detectors, FET devices, bipolar transistor devices, waveguide devices).

Another object of the invention is to provide fabrication methodologythat operates on the single layer structure to produce a heterojunctionthyristor device that can be adapted to operate as a laser, opticaldetector, optically (or electrically) controlled sampling switch, oroptical modulator.

Another object of the invention is to provide fabrication methodologythat operates on the single layer structure to produce complementaryHFET devices with n-channel and p-channel control elements respectively.

Another object of the invention is to provide fabrication methodologythat operates on the single layer structure to produce complementaryquantum-well-base bipolar transistors with n-channel and p-channelcontrol elements respectively.

Another object of the invention is to provide vertical cavityoptoelectronic devices in addition to lasers, detectors, modulators,amplifiers and switches that are interconnected by low loss passivewaveguides in the plane of the integrated circuit.

In accord with these objects, which will be discussed in detail below, asemiconductor device (and corresponding fabrication methodology)includes a novel series of layers formed on a substrate. The layersinclude a first plurality of layers including an n-type ohmic contactlayer, a second plurality of layers forming a p-type modulation dopedquantum well structure, a third plurality of layers forming an n-typemodulation doped quantum well structure, and a fourth plurality oflayers including a p-type ohmic contact layer. The first plurality oflayers preferably include a first etch stop layer that is used to formcontacts to the n-type ohmic contact layer. The fourth plurality oflayers preferably include a second etch stop layer that is used to formcontacts to the n-type modulation doped quantum well. Undoped spacerlayers are preferably disposed between the first and second plurality oflayers, between the second and third plurality of layers, and betweenthe third and fourth plurality of layers.

Preferably, each such etch stop layer is made sufficiently thin toenable current tunneling therethrough during operation ofoptoelectronic/electronic devices realized from this structure(including heterojunction thyristor devices, n-channel HFET devices,p-channel HFET devices, p-type quantum-well-base bipolar transistordevices, and n-type quantum-well-base bipolar transistor devices).

Electrodes that contact the n-type ohmic contact layer are formed by anetching operation that automatically stops at the first etch stop layer.Remaining portions of the first etch stop layer are removed to exposefirst areas of the n-type ohmic contact layer. A first metal layer isdeposited on the first areas of the n-type ohmic contact layer to formsuch electrodes.

Electrodes that contact the n-type quantum-well structure are formed byan etching operation that automatically stops at the second etch stoplayer. Remaining portions of the second etch stop layer are removed toexpose second areas of a layer thereunder. N-type ions are implantedinto these second areas to form at least one n-type implant region thatis operably coupled to the n-type modulation doped quantum wellstructure. At least one metal layer is deposited on the n-type implantregion to form such electrodes.

The etch stop layer(s) preferably comprise AlAs that functions as anetch stop during etching by a chlorine-based gas mixture that includesfluorine. The series of layers may comprise group III-V materials orstrained silicon heterostructures employing silicon-germanium (SiGe)layers.

In another aspect of the present invention, a high performance bipolartransistor device is realized from this structure by implanting p-typeions in a interdigitization pattern that forms a plurality of p-type ionimplant regions on both sides of the p-type modulation doped quantumwell structure to a depth that penetrates the n-type ohmic contactlayer. A base terminal electrode is operably coupled to the p-typemodulation doped quantum well structure by the plurality of p-type ionimplant regions. An emitter terminal electrode is operably coupled tothe n-type ohmic contact layer. The emitter terminal electrode comprisesa patterned metal layer formed on regions of the n-type ohmic contactlayer, such regions including portions between the p-type implantregions. The interdigitization pattern of the p-type implants reducescapacitance between the p-type modulation doped quantum well structureand the n-type ohmic contact layer to enable higher frequency operation.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional schematic showing a layer structure inaccordance with the present invention, and from which devices of thepresent invention can be made;

FIG. 1B is a schematic showing an exemplary layer structure made withgroup III-V material in accordance with the present invention, and fromwhich devices of the present invention can be made;

FIG. 1C shows the energy band diagram of the structure of FIG. 1B;

FIG. 2A is a cross-sectional schematic view showing the generalizedconstruction of an exemplary heterojunction thyristor device formed fromthe layer structure of FIG. 1A;

FIG. 2B is a cross-sectional schematic view showing the generalizedconstruction of an exemplary heterojunction thyristor device formed fromthe layer structure of FIG. 2A;

FIG. 2C is a pictorial illustration of an exemplary configuration of theheterojunction thyristor device of the present invention as anoptoelectronic/electronic device;

FIG. 2D is a graph showing the current-voltage characteristics of theheterojunction thyristor device in the NON-Conducting/OFF state ofoperation and the Conducting/ON state of operation, and the operationalconditions that cause the heterojunction thyristor device to switchbetween the OFF state of operation and the ON state of operation;

FIG. 3A is a cross-sectional schematic view showing the generalizedconstruction of an exemplary n-channel field effect transistor (FET)device formed from the layer structure of FIG. 1A;

FIG. 3B is a cross-sectional schematic view showing the generalizedconstruction of an exemplary n-channel field effect transistor (FET)device formed from the layer structure of FIG. 2A;

FIG. 3C is a pictorial illustration of an exemplary configuration of then-channel FET devices of FIGS. 3A and 3B;

FIG. 3D is a graph showing the generalized current-voltagecharacteristics of the n-channel FET devices of FIGS. 3A and 3B;

FIG. 4A is a cross-sectional schematic view showing the generalizedconstruction of an exemplary p-channel field effect transistor (FET)device formed from the layer structure of FIG. 1A;

FIGS. 4B and 4C are cross-sectional schematic views showing thegeneralized construction of exemplary p-channel field effect transistor(FET) devices formed from the layer structure of FIG. 2A;

FIG. 4D is a pictorial illustration of an exemplary configuration of thep-channel FET devices of FIGS. 4A, 4B and 4C;

FIG. 4E is a graph showing the generalized current-voltagecharacteristics of the p-channel FET devices of FIGS. 4A, 4B and 4C;

FIG. 5A is a cross-sectional schematic view showing the generalizedconstruction of an exemplary p-type quantum-well-base bipolar transistordevice formed from the layer structure of FIG. 1A;

FIGS. 5B and 5C are cross-sectional schematic views showing thegeneralized construction of exemplary p-type quantum-well-base bipolartransistor devices formed from the layer structure of FIG. 2A;

FIG. 5D is a pictorial illustration of an exemplary configuration of thep-type quantum-well-base bipolar transistor devices of FIGS. 5A, 5B and5C;

FIG. 5E is a graph showing the generalized current-voltagecharacteristics of the p-type quantum-well-base bipolar transistordevices of FIGS. 5A, 5B and 5C;

FIG. 6A is a cross-sectional schematic view showing the generalizedconstruction of an exemplary n-type quantum-well-base bipolar transistordevice formed from the layer structure of FIG. 1A;

FIG. 6B is a cross-sectional schematic view showing the generalizedconstruction of an exemplary n-type quantum-well-base bipolar transistordevices formed from the layer structure of FIG. 2A;

FIG. 6C is a pictorial illustration of an exemplary configuration of then-type quantum-well-base bipolar transistor devices of FIGS. 6A and 6B;and

FIG. 6D is a graph showing the generalized current-voltagecharacteristics of the n-type quantum-well-base bipolar transistordevices of FIGS. 6A and 6B.

FIG. 7 is a flow chart illustrating an exemplary method of fabricatingthe multilayer structure of FIG. 1A to integrate the variousoptoelectronic/electronic devices described herein on a commonsubstrate.

FIGS. 8A, 8B1, 8B2 8C1, and 8C2 illustrate two exemplary p-typequantum-well-base transistor devices that are realized byinterdigitization of the P+-type implants 171 on both sides of thecollector metal layer 174; FIG. 8A is a top view of the device structurefor the two exemplary p-type quantum-well-base transistor devices; FIG.8B1 illustrates the cross-section A-A of FIG. 8A of the first exemplaryp-type quantum-well-base transistor device; FIG. 8B2 illustrates thecross-section B-B of FIG. 8A of the first exemplary p-typequantum-well-base transistor device; FIG. 8C1 illustrates thecross-section A-A of FIG. 8A of the second exemplary p-typequantum-well-base transistor device; and FIG. 8C2 illustrates thecross-section B-B of FIG. 8A of the second exemplary p-typequantum-well-base transistor device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention builds upon novel device structures utilizingmodulation-doped QW heterojunctions that do not suffer from the problemsassociated with the prior art PHEMT devices and HBT. Such novel devicestructures are described in detail in U.S. Pat. No. 6,031,243; U.S.patent application Ser. No. 09/556,285 , filed on Apr. 24, 2000; U.S.patent application Ser. No. 09/798,316 , filed on Mar. 2, 2001; U.S.patent application Ser. No. 08/949,504 , filed on Oct. 14, 1997, U.S.patent application Ser. No. 10/200,967 , filed on Jul. 23, 2002; U.S.application Ser. No. 09/710,217 , filed on Nov. 10, 2000; U.S. PatentApplication No. 60/376,238 , filed on Apr. 26, 2002; and U.S.application Ser. No. 10/280,892 , filed on Oct. 25, 2002; each of thesereferences herein incorporated by reference in its entirety.

Turning now to FIG. 1A, a multi-layer sandwich structure in accordancewith the present invention, and from which devices of the presentinvention can be made, includes a bottom dielectric distributed braggreflector (DBR) mirror 12 formed on a substrate 10. The bottom DBRmirror 12 typically is formed by depositing pairs of semiconductor ordielectric materials with different refractive indices. When twomaterials with different refractive indices are placed together to forma junction, light will be reflected at the junction. The amount of lightreflected at one such boundary is small. However, if multiplejunctions/layer pairs are stacked periodically with each layer having aquarter-wave (¼n) optical thickness, the reflections from each of theboundaries will be added in phase to produce a large amount of reflectedlight (e.g., a large reflection coefficient) at the particular centerwavelength λ_(D). Deposited upon the bottom DBR mirror 12 is the activedevice structure which consists of two HFET devices. The first of theseis a p-channel HFET 11 (comprising layers 14,16,18,20 and 22) which hasone or more p-type modulation doped QWs and is positioned with the gateterminal on the lower side (i.e. on the bottom DBR mirror 12) and thecollector terminal on the upper side. The second of these is ann-channel HFET 13 (comprising layers 22,24,26,28,30) which has one ormore n-type modulation doped QWs and is positioned with the gateterminal on the top side and the collector terminal on the lower sidewhich is the collector of the p-channel device. Therefore a non-invertedN-channel device is stacked upon an inverted p-channel device to formthe active device structure.

The active device layer structure begins with n-type ohmic contactlayer(s) 14 which enables the formation of ohmic contacts thereto.Deposited on layer 14 is an n-type layer 16 which serves as an etch stoplayer when forming contacts to the ohmic contact layer(s) 14. Layer 16is also made sufficiently thin to enable current tunneling therethroughduring operation of optoelectronic/electronic devices realized from thisstructure. The thickness of layer 16 may be adjusted to set the desiredcurrent gain of a p-type quantum-well-base bipolar transistor realizedfrom this structure as described below. Preferably, the doping of thislayer 16 is such that it should not be depleted in any range ofoperation of the device, i.e. the total doping in this layer shouldexceed the total doping charge contained in the modulation doped layerof the p-type modulation doped QW structure 20 described below. In thisconfiguration, layer 14 achieves low contact resistance and layer 16defines the capacitance of the p-channel HFET 11 with respect to thep-type modulation doped QW heterostructure 20. This layer 16 also servesoptically as a small part of the lower waveguide cladding for opticaldevices realized in this structure. Note that a majority of the lowerwaveguide cladding is provided by the lower DBR mirror 12 itself.Deposited on layer 16 is an undoped layer 18. The undoped layer 18preferably includes a thin undoped capping layer 18 a and an undopedspacer layer 18 b. Capping layer 18 a serves to prevent oxidation oflayer 16 during subsequent manufacturing operations. Layers 14, 16 and18 serve electrically as part of the gate of the p-channel HFET 11.Deposited on layer 18 is a p-type modulation doped QW structure 20 thatdefines one or more quantum wells (which may be formed from strained orunstrained heterojunction materials). Deposited on the p-type modulationdoped QW structure 20 is an undoped spacer layer 22, which forms thecollector of the P-channel HFET device 11. All of the layers grown thusfar form the P-channel HFET device 11 with the gate ohmic contact on thebottom.

Undoped spacer layer 22 also forms the collector region of the N-channelHFET device 13. Deposited on layer 22 is a n-type modulation doped QWstructure 24 that defines one or more quantum wells (which may be formedfrom strained or unstrained heterojunction materials). Deposited on then-type modulation doped QW structure 24 is an undoped layer 26, whichpreferably includes an undoped spacer layer 26 a and a thin undopedcapping layer 26 b. Capping layer 26 b serves to prevent oxidation oflayer 26 a during subsequent manufacturing operations. Deposited onlayer 26 is a p-type layer structure 28, which includes layer 28 a andat least one p-type layer 28 b. Layer 28 a serves as an etch stop layerwhen forming contacts to the n-type inversion channel(s) of the NHFETdevice 13. Layer 28 a is also made sufficiently thin to enable currenttunneling therethrough during operation of optoelectronic/electronicdevices realized from this structure. The thickness of layer 28 a may beadjusted to set the desired current gain of an n-type quantum-well-basebipolar transistor realized from this structure as described below.Preferably, the doping of this layer 28 a is such that it should not bedepleted in any range of operation of the device, i.e. the total dopingin this layer should exceed the total doping charge contained in themodulation doped layer of the n-type modulation doped QW structure 24described above. Layer structure 28 also serves optically as a smallpart of the upper waveguide cladding for optical devices realized inthis structure. Note that a majority of the upper waveguide cladding isprovided by the upper DBR mirror (not shown). Deposited on the p-typelayer structure 28 is a p-type ohmic contact layer(s) 30 which enablesthe formation of ohmic contacts thereto. Layers 26, 28 and 30 serveelectrically as part of the gate of the n-channel HFET 13. In thisconfiguration, layer 30 achieves low contact resistance and layer 28 adefines the capacitance of the n-channel HFET 13 with respect to then-type modulation doped QW heterostructure 24.

Alternatively, the active device structure may be described as a pair ofstacked quantum-well-base bipolar transistors formed on the bottom DBRmirror 12. The first of these is an n-type quantum-well-base bipolartransistor (comprising layers 14, 16, 18, 20 and 22) which has one ormore p-type modulation doped quantum wells and is positioned with theemitter terminal on the lower side (i.e. on the mirror as justdescribed) and the collector terminal on the upper side. The second ofthese is an n-type quantum-well-base bipolar transistor (comprisinglayers 22, 24, 26, 28, 30) which has one or more n-type modulation dopedquantum wells and is positioned with the emitter terminal on the topside and the collector terminal on the lower side which is the collectorof the p-type quantum-well-base bipolar transistor. Therefore anon-inverted n-channel device is stacked upon an inverted p-channeldevice to form the active device structure. In this configuration, thegate terminal of the p-channel HFET device 11 corresponds to the emitterterminal of the p-type quantum-well-base bipolar transistor, the p-typeQW structure 20 corresponds to the base region of the p-typequantum-well-base bipolar transistor, spacer layer 22 corresponds to thecollector region of both the p-type quantum-well-base bipolar transistorand the n-type quantum-well-base bipolar transistor, the n-type QWstructure 24 corresponds to the base region of the n-typequantum-well-base bipolar transistor, and the gate terminal of then-channel HFET device 13 corresponds to the emitter electrode of then-type quantum-well-base bipolar transistor.

To form a resonant cavity device where light is input into and emittedfrom the device laterally (i.e., from a direction normal to the crosssection of FIG. 1A), a diffraction grating and top DBR mirror are formedover the active device structure described above. For vertical cavitylasing devices, the diffraction grating performs the function ofdiffracting light produced by the vertical cavity into light propagatinglaterally in a waveguide which has the top DBR mirror and bottom DBRmirror as waveguide cladding layers and which has lateral confinementregions (typically formed by implants as described herein in moredetail). For vertical cavity detecting devices, the diffraction gratingperforms the function of diffracting incident light that is propagatingin the lateral direction into the vertical cavity mode, where it isabsorbed resonantly in the vertical cavity.

Alternatively, light may enter and exit the resonant vertical cavitythrough an optical aperture (not shown) in the top surface of thedevice. In this case, the diffraction grating is omitted, the top DBRmirror defines a cavity for the vertical emission and absorption oflight, and the device operates as a vertical cavity surface emittinglaser/detector.

The distance between the top DBR mirror and bottom DBR mirror preferablyrepresents an integral number of ½ wavelengths at the designatedwavelength. This distance is controlled by adjusting the thickness ofone or more of the layers therebetween to enable this condition.

The multilayer structure described above may be realized with a materialsystem based on group III-V materials (such as a GaAs/AlGaAs).Alternatively, strained silicon heterostructures employingsilicon-germanium (SiGe) layers may be used to realize the multilayerstructures described herein. FIG. 1B illustrates an exemplary epitaxialgrowth structure utilizing group III-V materials for realizing themultilayer structure of FIG. 1A and theoptoelectrical/electrical/optical devices formed from this structure inaccordance with the present invention. The structure of FIG. 1B can bemade, for example, using known molecular beam epitaxy (MBE) techniques.As shown, a first semiconductor layer 151 of AlAs and a secondsemiconductor layer 152 of GaAs are alternately deposited (withpreferably at least seven pairs) upon a semi-insulating gallium arsenidesubstrate 149 in sequence to form the top dielectric distributed braggreflector (DBR) mirror 12. The number of AlAs layers will preferablyalways be one greater than the number of GaAs layers so that the firstand last layers of the mirror are shown as layer 151. In the preferredembodiment the AlAs layers 151 are subjected to high temperature steamoxidation to produce the compound Al_(x)O_(y) so that a mirror will beformed at the designed center wavelength. Therefore the thicknesses oflayers 151 and 152 in the mirror are chosen so that the final opticalthickness of GaAs and Al_(x)O_(y) are one quarter wavelength of thecenter wavelength λ_(D). Alternatively the mirrors could be grown asalternating layers of one quarter wavelength thickness of GaAs and AlAsat the designed wavelength so that the oxidation step is not used. Inthat case, many more pairs are required (with typical numbers such as 22pairs) to achieve the reflectivity needed for efficient lasing.

Deposited upon the mirror is the active device structure which consistsof two HFET devices. The first of these is the p-channel HFET (PHFET)11, which has one or more p-type modulation doped quantum wells and ispositioned with the gate terminal on the bottom (i.e. on the mirror 12just described) and the collector terminal above. The second of these isan n-channel HFET (NHFET) 13, which has one or more n-type modulationdoped quantum wells and is positioned with the gate terminal on top andthe collector terminal below. The collector region of the NHFET device13 also functions as the collector region of the PHFET device 11.However, the collector terminal of the NHFET device 13 is a p-typecontact to p-type quantum well(s) disposed below (above) the collectorregion, while the collector terminal of the PHFET device 11 is a n-typecontact to n-type quantum well(s) disposed above the collector region.Therefore a non-inverted n-channel device is stacked upon an invertedp-channel device to form the active device structure.

The active-device layer structure begins with layer 153 of N+ type GaAsthat enables the formation of ohmic contacts thereto (for example, whencontacting to the cathode terminal of a heterojunction thyristor device,the gate terminal of an inverted p-channel HFET device, thesub-collector terminal of an n-channel HFET device, or the emitterterminal of a p-type quantum-well-base bipolar device). Layer 153 has atypical thickness of 1000-2000 Å and a typical n-type doping of 3.5×10¹⁸cm⁻³. The N+ doped GaAs layer 153 corresponds to the ohmic contact layer14 of FIG. 1A. Deposited on layer 153 is a layer 166 a of n-type AlAshaving a typical thickness of 30-200 Å and a typical n-type doping of3.5×10 ¹⁸ cm⁻³. One constraint upon the thickness and the doping of thislayer 166 a is that it should not be depleted in any range of operationof the device, i.e. the total doping in this layer should exceed thetotal doping charge contained in the layer 155 c described below. Thislayer 166 a serves optically as a small part of the lower waveguidecladding of the device. Note that a majority of the lower waveguidecladding for waves propagating in the guide formed by the opticallyactive region of the device is provided by the lower DBR mirror itself.The lower DBR mirror causes the light to be guided partially as adielectric waveguide and partially as a mirror waveguide. In addition,layer 166 a also acts as an etch stop layer (described below in moredetail) when forming contacts to the ohmic contact layer 153. Anotherconstraint on the thickness of layer 166 a is that it must be madesufficiently thin to enable hole current to flow through it bytunneling. In this manner, the thickness of layer 166 a determines thecurrent gain of an inverted p-type quantum-well-base bipolar transistorrealized in this growth structure. Next is a layer 166 b of undoped GaAshaving a typical thickness of 6-20 Å. This layer 166 b serves to preventoxidation of the layer 166 a during subsequent oxidation operations(e.g., where the bottom DBR mirror layers 151/152 are oxidized). Inaddition, undoped GaAs layer 166 b is advantageous in a single aluminumeffusion cell MBE system because it accommodates a growth interruptionto change the growth temperature between layers 166 a and 155 b asrequired.

Next are three layers (155 b, 155 c, and 155 d) of Al_(x2)Ga_(1-x2)As.These three layers have a total thickness about 300-500 Å and where x2is about 15%. The first layer 155 b is about 200-300 Å thick and isundoped. The second layer 155 c is about 80 Å thick and is doped P+ typein the form of delta doping with a typical concentration of 3.5×10¹⁸cm⁻³. And the third layer 155 d is about 20-30 Å thick and is undoped.The layers 155 d and 166 b form the lower separate confinementheterostructure (SCH) layer for the laser, amplifier and modulatordevices. The N+ AlAs layer 166 a corresponds to the n-type layer 16 ofFIG. 1A, and the undoped GaAs layer 166 b and the undoped AlGaAs layer155 b corresponds to the undoped spacer layer 18 of FIG. 1A. To realizea p-type quantum-well-base bipolar transistor (and/or a p-channel HFET)with a cutoff frequency of about 40 GHz, the combined thickness oflayers 166 b and 155 b is preferably on the order of 300 Å. And torealize a p-type quantum-well-base bipolar transistor (and/or ap-channel HFET) with a cutoff frequency of about 90 GHz, the combinedthickness of layers 166 b and 155 b is preferably on the order of 250 Å.

The next layers define the quantum well(s) that form the inversionchannel(s) during operation of the PHFET 11. For a strained quantumwell, this consists of a spacer layer 156 of undoped GaAs that is about10-25 Å thick and then combinations of a quantum well layer 157 (that isabout 40-80 Å thick) and a barrier layer 158 of undoped GaAs. Thequantum well layer 157 may be comprised of a range of compositions. Inthe preferred embodiment, the quantum well is formed from aIn_(0.2)Ga_(0.8)AsN composition with the nitrogen content varying from0% to 5% depending upon the desired natural emission frequency. Thus,for a natural emission frequency of 0.98 μm, the nitrogen content willbe 0%; for a natural emission frequency of 1.3 μm, the nitrogen contentwill be approximately 2%; and for a natural emission frequency of 1.5μm, the nitrogen content will be approximately 4-5%. The well-barriercombination will typically be repeated (for example, three times asshown) to define the quantum wells that form the inversion channelsduring operation of the PHFET 11 (however single quantum well structuresare also possible). Unstrained quantum wells are also possible.Following the last barrier of undoped GaAs is a layer 167 of undopedGaAs and a layer 159 of undoped Al_(x2)Ga_(1-x2)As. The undoped GaAslayer 167 has a typical thickness of 250-500 Å, and the undopedAl_(x2)Ga_(1-x2)As layer 159 has a typical thickness of 0.5 μm. Theselayers 167 and 159 form the collector of the PHFET device 11. Thepurpose of the GaAs layer 167 is to accommodate a change in the growthtemperature from about 530° C. (as required for the InGaAs quantum wellstructure of layer 157) to about 610° C. (as required forAl_(x2)Ga_(1-x2)As layer 159). Layer 167 performs no electrical purposeand so it should be electrically totally transparent to all currentflows. Therefore, layer 167 is thin enough that currents may passthrough it by tunneling with negligible voltage drop. All of the layersgrown thus far form the PHFET device 11 with the gate contact on thebottom. The layers between the P+ AlGaAs layer 155 c and the lastundoped GaAs barrier layer 158 correspond to the p-type modulation dopedheterojunction QW structure 20 of FIG. 2A. Undoped GaAs layer 167 andundoped AlGaAs layer 159 correspond to the undoped spacer layer 22 ofFIG. 1A.

Layers 167 and 159 also form the collector region of the NHFET device13. Deposited on layer 159 is a layer 160 (shown as two sublayers 160 a,160 b) of undoped GaAs of about 200-250 Å total thickness, which formthe barrier of the first n-type quantum well. Layer 160 is thicker thanthe normal barrier layer (layer 160 b) of about 100 Å because itaccommodates the growth interruption to change the growth temperaturefrom 610° C. (as required for optical quality Al_(x2)Ga_(1-x2)As layers)to about 530° C. for the growth of InGaAs. The next layer 161 is thequantum well of In_(0.2)Ga_(0.8)As, which is undoped and about 40-80 Åin thickness. The quantum well layer 161 may be comprised of a range ofcompositions as described above with respect to the quantum well layer157. In the preferred embodiment, the quantum well is formed from anIn_(0.2)Ga_(0.8)AsN composition with the nitrogen content varying from0% to 5% depending upon the desired natural emission frequency. It isnoted that the n-type quantum well layer 161 need not be of the sameformulation as the p-type quantum well layer 157. The barrier-wellcombination (layers 160 b, 161) will typically be repeated (for example,three times as shown) to define the quantum wells that form theinversion channel(s) during operation of the NHFET 13. Then there is atop barrier layer 162 of about 10-30 Å of undoped GaAs whichaccommodates a growth interruption and a change of growth temperature.

Next there are three layers (163 a, 163 b, 163 c) of Al_(x2)Ga_(1-x2)Asof about 300-400 Å total thickness. These three layers include a spacerlayer 163 a of undoped Al_(x2)Ga_(1-x2)As that is about 20-30 Å thick, amodulation doped layer 163 b of N+ type doping of Al_(x2)Ga_(1-x2)As(with doping about 3.5×10¹⁸ cm⁻³) that is about 80 Å thick, and a spacerlayer 163 c of undoped Al_(x2)Ga_(1-x2)As that is about 200-300 Å thick.Next is a layer 168 a of undoped GaAs that is about 6-20 Å thick, and aP+ type doped layer 168 b of AlAs (with doping about 3.5×10¹⁸ cm⁻³) thatis about 300 Å. In contrast to layer 163 b which is always depleted,layer 168 b should never be totally depleted in operation (i.e., thetotal doped charge in layer 168 b should always exceed that in layer 163b). Layers 168 b and 163 b (and the undoped spacer layers 163 c and 168a therebetween) form the two plates of a parallel plate capacitor whichforms the field-effect input to all active devices. For theoptoelectronic device operation, layer 163 a is the upper SCH region.Layer 168 b also acts as a etch stop layer (described below in moredetail) when forming contacts to the N-type inversion channel(s) of theNHFET 13 (for example, when contacting to the N-channel injectorterminal(s) of a heterojunction thyristor device, the source/drainterminals of an n-channel HFET device, the base terminal of an n-typequantum-well-base bipolar transistor, or the collector terminal of ap-type quantum-well-base bipolar transistor). Moreover, similar to layer166 b, layer 168 a must be made sufficiently thin to enable electroncurrent to flow through it by tunneling. In this manner, the thicknessof this layer 168 a determines the current gain of a n-typequantum-well-base bipolar transistor device realized in this growthstructure. In addition, because layer 168 a is thin, it does not easilyoxidize during subsequent oxidation operations (e.g., where the bottomDBR mirror layers are oxidized). Further, undoped GaAs layer 168 a isadvantageous in a single aluminum effusion cell MBE system because itaccommodates a growth interruption to change the growth temperaturebetween layers 163 c and 168 b as required. The layers between theundoped GaAs barrier layer 160 a and the N+ AlGaAs layer 163 bcorrespond to the n-type modulation doped heterojunction QW structure 24of FIG. 1A. Undoped AlGaAs layer 163 c and undoped GaAs layer 168 acorresponds to the undoped spacer layer 26 of FIG. 1A. To realize ann-type quantum-well-base bipolar transistor (and/or an n-channel HFET)with a cutoff frequency of about 40 GHz, the combined thickness oflayers 163 c and 168 a is preferably on the order of 300 Å. To realizean n-type quantum-well-base bipolar transistor (and/or an n-channelHFET) with a cutoff frequency of about 90 GHz, the combined thickness oflayers 163 c and 168 a is preferably on the order of 250 Å.

A layer 164 of p-type GaAs is deposited next to form part of the upperwaveguide cladding layer for the laser, amplifier and modulator devices.Note that a majority of the upper waveguide cladding for wavespropagating in the guide formed by the optically active region of thedevice is provided by the upper DBR mirror itself. The upper DBR mirrorcauses the light to be guided partially as a dielectric waveguide andpartially as a mirror waveguide. Layer 164 also forms a spacer layer inwhich to accommodate the aperture implants which steers the current intothe VCSEL active region. It should provide a low resistance access tothe top contact. It has a typical thickness of 300 Å. The p-type layers168 b and 164 correspond to the p-type layer(s) 28 of FIG. 1A.

Deposited next is an ohmic contact layer 165 (which may comprise asingle layer of GaAs or a combination of GaAs (165 a) and InGaAs (165 b)as shown). In the illustrative embodiment shown, the GaAs layer 165 a isabout 50-100 Å thick and doped to a very high level of P+ type doping(about 1×10²⁰ cm⁻³) and the InGaAs layer 165 b is about 25-50 Å thickand doped to a very high level of P+ type doping (about 1×10²⁰ cm⁻³) toenable the best possible ohmic contact.

Alternatively, the active device structure may be described as a pair ofstacked quantum-well-base bipolar transistors formed on the bottom DBRmirror (layers 151/152). The first of these is a p-typequantum-well-base bipolar transistor (comprising layers 153 through 159)which has one or more p-type modulation doped quantum wells and ispositioned with the emitter terminal on the lower side (i.e. on themirror as just described) and the collector terminal on the upper side.The second of these is an n-type quantum-well-base bipolar transistor(comprising layers 159 through 165 b) which has one or more n-typemodulation doped quantum wells and is positioned with the emitterterminal on the top side and the collector terminal on the lower sidewhich is the collector of the p-type quantum-well-base bipolartransistor. Therefore a non-inverted n-channel device is stacked upon aninverted p-channel device to form the active device structure. In thisconfiguration, the gate terminal of the PHFET 11 corresponds to theemitter terminal of the p-type quantum-well-base bipolar transistor, thep-type QW structure (layers 155 c though 158) corresponds to the baseregion of the p-type quantum-well-base bipolar transistor, spacer layer159 corresponds to the collector region of both the p-typequantum-well-base bipolar transistor and the n-type quantum-well-basebipolar transistor, the n-type QW structure (layers 160 a through 163 b)corresponds to the base region of the n-type quantum-well-base bipolartransistor, and the gate terminal of the NHFET 13 corresponds to theemitter electrode of the n-type quantum-well-base bipolar transistor.

The band diagram of the FIG. 1B structure is shown in FIG. 1C.

To form a resonant cavity device where light is input into and emittedfrom the device laterally (i.e., from a direction normal to the crosssection of FIG. 1B), a diffraction grating (for example, as described indetail in U.S. Pat. No. 6,031,243) and top DBR mirror is formed over theactive device structure described above. For vertical cavity lasingdevices, the diffraction grating performs the function of diffractinglight produced by the vertical cavity into light propagating laterallyin a waveguide which has the top DBR mirror and bottom DBR mirror aswaveguide cladding layers and which has lateral confinement regions(typically formed by implants as described herein in more detail). Forvertical cavity detecting devices, the diffraction grating performs thefunction of diffracting incident light that is propagating in thelateral direction into the vertical cavity mode, where it is absorbedresonantly in the vertical cavity.

Alternatively, light may enter and exit the resonant vertical cavityvertically through an optical aperture in the top surface of the device.In this case, the diffraction grating is omitted, the top DBR mirrordefines a cavity for the vertical emission and absorption of light, andthe device operates as a vertical cavity surface emittinglaser/detector. The distance between the top DBR mirror and bottom DBRmirror preferably represents an integral number of ½ wavelengths at thedesignated wavelength. Preferably, the thickness of layer 164 and/orlayer 159 is adjusted to enable this condition.

The structure of FIGS. 1A and 1B may also be used to realize variousoptoelectronic devices, including heterojunction thyristor devices, anarray of transistor devices (including n-channel HFET devices, p-channelHFET devices, n-type quantum-well-base bipolar transistors and p-typequantum-well-base bipolar transistors), and waveguide devices.

FIG. 2A illustrates an exemplary heterojunction thyristor devicerealized from the multilayer sandwich of FIG. 1A. As shown, one or moreanode terminal electrodes (two shown as 36A and 36B) are operablycoupled to the p-type ohmic contact layer 30, one or more n-channelinjector terminal electrodes (two shown as 38A, 38B) are operablycoupled to the n-type QW structure 24, one or more p-channel injectorterminal electrodes (two shown as 38C, 38D) are operably coupled to thep-type QW structure 20, and one or more collector terminal electrodes(two shown as 40A, 40B) are operably coupled to the n-type ohmic contactlayer 14. When forming the heterojunction thyristor device via etchingand metallization, etch stop layer 28 a is used as an etch stop in orderto form a contact that is electrically coupled to the n-type QWstructure 24, which is subsequently metallized to form the n-channelinjector terminal(s) (38A, 38B) that are electrically coupled to then-type QW structure 24; and layer 16 is used as an etch stop layer inorder to form a contact that is electrically coupled to the n-type ohmiccontact layer 14, which is subsequently metallized to form the cathodeterminal electrode(s) (40A, 40B) of the device.

In alternative embodiments, the p-channel injector terminals (38C, 38D)may be omitted. In such a configuration, the N-channel injectorterminals (38A, 38B), which are coupled to the n-type inversion QWstructure 24 are used to control charge in such n-type inversion QWchannel(s) as described herein. In yet another alternative embodiment,the N-channel injector terminals (38A, 38B) may be omitted. In such aconfiguration, the p-channel injector terminals (38C, 38D), which arecoupled to the p-type inversion QW structure 20 are used to controlcharge in such p-type inversion QW channel(s) as described herein.

FIG. 2B illustrates an exemplary heterojunction thyristor devicerealized from the multilayer sandwich of FIGS. 1B and 1C. To connect tothe anode terminal of the device, alignment marks (not shown) aredefined by etching, and then a layer of Si₃N₄ or Al₂O₃ or other suitabledielectric (not shown) is deposited to act as protection for the surfacelayer and as a blocking layer for subsequent ion implants. Preferably,this dielectric layer also forms the first layer of the top DBR mirror.Then an ion implant 175 of n-type is performed using a photomask that isaligned to the alignments marks, and an optical aperture is defined bythe separation between the implants 175. The implants 175 create a p-njunction in the layers between the n-type quantum well(s) and thesurface, and the aperture between the implants defines the region inwhich the current may flow, and therefore the optically active region177 as shown. The current cannot flow into the n-type implanted regions175 because of the barrier to current injection. The current flowtrajectory is shown in FIG. 2B as arrows. For lasing applications, thelaser threshold condition is reached before the voltage for turn-on ofthis barrier. Following the implant 175, a metal layer 174 (preferablycomprising tungsten) is deposited and defined to form anode terminals36A and 36B (which collectively form the anode terminal 36) of thedevice.

Then an ion implant 170 of n+-type is performed using the metal 174 as amask that is self-aligned to the metal features, to thereby formcontacts to the n-type QW inversion channel(s). During this operation, achlorine-based gas mixture that includes fluorine is used as an etchantto etch down to the etch-stop layer 168 b. The etch rate through theInGaAs layer 165 b and GaAs layers (165 a and 164) is fairly rapid.However, because of the presence of fluorine in the etchant, the etchrate decreases drastically when the AlAs layer 168 b is encountered.This is because the AlAs layer 168 b has a high percentage of Aluminum,which forms AlF in the presence of the etch mixture. The AlF deposits onthe surface of the structure and prevents further etching (because it isnon-volatile and not etched by any of the conventional etchants). Inthis manner, the AlAs layer 168 b operates as an etch stop layer. Thislayer is then easily dissolved in de-ionized (DI) water or wet bufferedhydrofluoric acid (BHF) to form mesas at the undoped GaAs layer 168 a.The resulting mesas at the undoped GaAs layer 168 a are subject to theN+ ion implants 170, which contact the n-type QW inversion channel(s).

Then an ion implant 171 of p+-type is performed using a photomask thatis aligned to the alignments marks, to thereby form contacts to thep-type QW inversion channel(s). During this operation, a masking step isperformed to protect all devices with N+ type inversion channels and thesemiconductor is etched down to the bottom of layer 22 in FIG. 2A. Thismask is aligned to metal features 174. Then the resulting mesas aresubject to P+ ion implants 171, which electrically contact the P-type QWinversion channel(s).

In alternative embodiments, the P+ ion implants 171 (and correspondingP-channel injector terminals 38C and 38D) may be omitted. In such aconfiguration, the N-channel injector terminals 38A and 38B (which arecoupled to the n-type inversion QW channel(s) of the NHFET device 13 bythe N+ ion implants 170) are used to control charge in such n-typeinversion QW channel(s) as described herein. In yet another alternativeembodiment, the N+ ion implants 170 (and corresponding N-channelinjector terminals 38A and 38B) may be omitted. In such a configuration,the P-channel injector terminals 38C and 38D (which are coupled to thep-type inversion QW channel(s) of the PHFET 11 device by the P+ ionimplants 171) are used to control charge in such p-type inversion QWchannel(s) as described herein.

Connection to the cathode terminal (e.g., N+ layer 153) of the device ismade by etching with a chlorine-based gas mixture that includesfluorine. This etch is performed down to the AlAs etch stop layer 166 a.This layer 166 a is then easily dissolved in de-ionized (DI) water orwet buffered hydrofluoric acid (BHF) to form resulting mesas in the N+layer 153. Next the device is subjected to a rapid thermal anneal (RTA)of the order of 900° C. or greater to activate all implants. Then thedevice is isolated from other devices by an etch down to thesemi-insulating substrate 149, which includes an etch through the mirrorpairs 151/152 of AlAs/GaAs. At this point, the device is oxidized in asteam ambient to create layers 179/180, which form the top DBR mirror.During this oxidation step, the exposed sidewalls of the etched AlGaAslayers are passivated by the formation of very thin layers of oxide. Thefinal step in the fabrication is the deposition (preferably via liftoff) of metal contacts. These contacts come in three forms. One is themetal layer 176 (preferably comprising an n-type Au alloy metal such asAuGe/Ni/Au) deposited on the N+ type implants 170 to form the N-channelinjector terminal electrodes 38A, 38B. The second is the metal layer 178(preferably comprising an p-type Au metal alloy such as AuZn/Cr/Au)deposited on the P+ type implant 171 to form the p-channel injectorterminal electrodes 38C, 38D. The third is the metal layer 181(preferably comprising an n-type Au alloy metal such as AuGe/Ni/Au)deposited on the mesas at the N+ layer 153 to form the cathode terminalelectrodes 40A, 40B of the device.

To form a device suitable for in-plane optical injection into a resonantvertical cavity and/or in-plane optical emission from the resonantvertical cavity, a diffraction grating 32 (for example, as described indetail in U.S. Pat. No. 6,031,243) and top DBR mirror is formed inconjunction with the active device structure as described above. To forma device suitable for vertical optical injection into (and/or opticalemission from) a resonant vertical cavity, the diffraction grating 32 isomitted. The top DBR mirror is preferably created by the deposition ofone or more dielectric layer pairs (179,180), which typically compriseSiO₂ and a high refractive index material such as GaAs, Si, or GaN.

FIGS. 2C and 2D illustrate the operational characteristics of theheterojunction thyristor devices of FIGS. 2A and 2B. The device switchesfrom a non-conducting/OFF state (where the current I is substantiallyzero) to a conducting/ON state (where current I is substantially greaterthan zero) when: i) the anode terminal 36 is forward biased (e.g. biasedpositively) with respect to the cathode terminal 40; and ii) opticalenergy is supplied and resonantly absorbed in the QW channel(s) of thedevice and/or electrical energy is injected via the injector terminal 38into the QW channel(s) of the device such that charge in the QWchannel(s) is greater than the critical switching charge Q_(CR), whichis that charge that reduces the forward breakdown voltage such that nooff state bias point exists. The critical switching charge Q_(CR) isunique to the geometries and doping levels of the device. The deviceswitches from the conducting/ON state to the non-conducting/OFF statewhen the charge in the QW channel(s) of the device decreases below theholding charge Q_(H), which is the critical value of the channel chargewhich will sustain holding action.

As an optoelectronic component, the heterojunction thyristor devices ofthe present invention are multifunctional. For example, the devices canbe configured to operate as a laser by biasing the device such that thecurrent I in the conducting/ON state is above the threshold for lasingI_(L) as shown in FIG. 2D. In such a configuration, the lasing actionproduces an output optical signal that is emitted from the device and acorresponding output electrical signal as shown in FIG. 2C. Such lasingaction can be triggered by an optical control signal resonantly absorbedin the QW channel(s) of the device and/or an electrical control signalinjected into the QW channel(s) of the device.

The heterojunction thyristor devices of the present invention can alsobe configured to operate as an optical detector by biasing the devicessuch that incident light will be resonantly absorbed and switch thedevice into its ON state, which produces an output electrical signal asshown in FIG. 2C. In the ON state, the device may produce acorresponding output optical signal via lasing action if the device isbiased such that the current I in the ON state is above the thresholdfor lasing I_(L).

In addition, the heterojunction thyristor devices of the presentinvention can be configured to operate as an optically-controlled (orelectrically-controlled) sampling device (e.g., sampling switch) whereinan input terminal is selectively coupled to an output terminal inresponse to an optical control signal (or an electrical control signal).The input terminal and output terminal correspond to the n-channelinjector terminal pair (or p-channel injector terminal pair) of thedevices shown in FIGS. 2A and 2B. For optical control, theheterojunction thyristor device is biased such that the optical controlsignal is resonantly absorbed by the device and switches the devicebetween the ON state/OFF state. For electrical control, theheterojunction thyristor device is biased such that the electricalcontrol signal is injected into the QW channel(s) of the device andswitches the device between the ON state/OFF state. In the ON state, then-channel injector terminal pair (or p-channel injector terminal pair)are operably coupled together (with minimal potential voltage differencetherebetween). In the OFF state, the n-channel injector terminal pair(or p-channel injector terminal pair) are electrically isolated from oneanother.

In addition, the heterojunction thyristor devices of the presentinvention can be configured to operate as various other optoelectroniccomponents including a digital optical modulator and optical amplifieras described below.

A digital optical modulator operates in one of two distinct opticalstates in modulating an input optical signal. In optical state 1, thereis substantially no loss to the input optical signal via absorption. Inoptical state 2, substantially all of the input optical signal isabsorbed. To configure the heterojunction thyristor device as a digitaloptical modulator, an optical path is provided through the device eithervertically or in the waveguide mode, and an input signal is applied tothe injector terminal 38. When the input signal produces a forward biasbetween the injector terminal 38 and the anode terminal 36 sufficient toproduce charge in the QW channel(s) of the device greater than thecritical switching charge Q_(CR), the heterojunction thyristor deviceoperates in its conducting/ON state. The device is biased such that thecurrent I through the device in the ON state is substantially below thethreshold for lasing (preferably about 0.5 to 0.7 of the lasingthreshold current). In this configuration, in the ON state, the deviceoperates in optical state 1 whereby there is substantially no loss tothe input optical signal via absorption. When the input signal producesa reverse bias between the injector terminal 38 and the anode terminal36, charge is drawn from the injector terminal 38 such that the channelcharge in the QW channel(s) of the device falls below the hold chargeQ_(H), and the heterojunction thyristor device operates in itsnon-conducting/OFF state. In the OFF state, the device operates inoptical state 2 whereby substantially all of the input optical signal isabsorbed. Preferably, the digital optical modulator includes adiffraction grating as described above. This grating enhances theabsorption and enables modulation between the 0 and 1 states in theshortest possible length.

An optical amplifier amplifies an input optical signal to produce acorresponding output optical signal with an increased intensity level.To configure the heterojunction thyristor device as an opticalamplifier, a forward bias is applied between the injector terminal 38and cathode terminal 40, and a forward bias is applied between the anodeterminal 36 and cathode terminal 40 through a load resistance R_(L) thatsets the current I in the ON state at a point substantially below lasingthreshold I_(L). In this configuration, in the ON state, the deviceamplifies an input optical signal to produce a corresponding outputoptical signal with an increased intensity level. The optical amplifiermay be switched into and out of the ON state by applying forward andreverse biases to the injector terminal 38 with respect to the anodeterminal 36 as described above. The gain of the optical amplifier in theON state and thus the output signal intensity level may be changed byadjusting the current I in the ON state. Preferably, the opticalamplifier operates without the existence of a diffraction grating in thestructure. In this configuration, there will be no interaction betweenthe waveguide traveling wave and the vertical cavity oscillation. Thegain is obtained by using the high density of electrons and holes in thevertical laser above threshold.

The structures of FIGS. 1A and 1B may also be used to produce anin-plane passive waveguide. In such a configuration, the diffractiongrating, the ohmic gate/emitter electrode layers, and any contacts to n+and p+ regions are omitted in order to minimize waveguide loss. Thewaveguide ridge cross-section is formed by a combination of severalmesas, which are formed by vertical/horizontal surfaces formed in thelayers between the top DBR mirror and the bottom DBR mirror, to provideboth laterally guiding and vertical guiding of light therein.

In addition, the multilayer structure of FIGS. 1A and 1B can be used torealize various other optoelectronic components including a PIN detectorand analog optical modulator as described below.

A PIN detector generates an electrical signal proportional to theoptical signal incident thereon. To configure the multilayer structureof FIGS. 1A and 1B as a PIN detector, the n-type ohmic contact layer(which is coupled to the cathode terminal 40 of the heterojunctionthyristor device) floats electrically and a reverse bias is appliedbetween the p-type ohmic contact layer 30 (which is coupled to the anodeterminal 36 of the heterojunction thyristor device) and the n-channelinjector terminal(s) (38A, 38B). Such a configuration creates areverse-bias PIN junction that generates an electrical signal(photocurrent) proportional to the optical signal incident to thevertical cavity. Preferably, the PIN detector incorporates a diffractiongrating for efficient operation.

An analog optical modulator modulates an input optical signal linearlyover a range of modulation values. To configure the multilayer structureof FIGS. 1A and 1B as an analog optical modulator, the n-type ohmiccontact layer (which is coupled to the cathode terminal 40 of theheterojunction thyristor device) floats electrically. Similar to theheterojunction thyristor device, an optical path is provided through thedevice either vertically or in the waveguide mode, and an input signalis applied to the anode terminal 36 with respect to the injectorterminal(s) 38 such that the anode terminal 36 is biased positively withrespect to the injector terminal(s) 38. In this configuration, thevoltage at the anode terminal 36 is varied over a range of voltagelevels where absorption of the device varies linearly. The top of thevoltage range (where minimum absorption occurs) is defined by theoperation point where conduction occurs from the anode terminal 36 tothe injector terminal(s) 38. Preferably, the analog modulatorincorporates a diffraction grating for efficient operation.

The structure of FIGS. 1A and 1B may also be used to realize varioustransistor devices, including n-channel HFET devices, p-channel HFETdevices, n-type quantum-well-base bipolar transistors and p-typequantum-well-base bipolar transistors as described below in detail.

FIG. 3A illustrates an exemplary n-channel HFET device realized from themultilayer sandwich of FIG. 1A. As shown, a source terminal electrode 42and a drain terminal electrode 44 are electrically coupled to the n-typeQW structure 24 to form a channel region therebetween. A gate terminalelectrode 46 is formed on the p-type ohmic contact layer 30 and coversthe n-type QW inversion channel. Preferably, one or more collectorterminal electrodes 48 are electrically coupled to the p-type QWstructure 20 below the n-type QW inversion channel. When forming then-channel HFET device via etching and metallization, etch stop layer 28a is used as an etch stop in order to form contacts that areelectrically coupled to the n-type QW structure 24 (such contacts aresubsequently metallized to form the source terminal electrode 42 and thedrain terminal electrode 44). In this configuration, the collectorterminal electrode 48 is preferably connected as a back gate similar tothe substrate contact in a silicon-based MOSFET transistor.

FIG. 3B illustrates an exemplary n-channel HFET device realized from themultilayer sandwich of FIGS. 1B and 1C. As shown, a metal layer 174(preferably comprising tungsten) is deposited on the ohmic contact layer165 b to form the gate terminal electrode 46. The structure outside thegate terminal electrode 46 is etched down to the etch stop layer 168 b.During this operation, a chlorine-based gas mixture that includesfluorine is used as an etchant to etch down to the etch-stop layer 168b. This etch stop layer 168 b is then easily dissolved in de-ionized(DI) water or wet buffered hydrofluoric acid (BHF) to form mesas at theundoped GaAs layer 168 a. The resulting mesas at the undoped GaAs layer168 a are then subject to N+ ion implants 170, which are electricallycoupled to the n-type QW inversion channel(s). On the source terminalelectrode side of the device, the resulting structure is etchedpreferably down to layer 158, and the resulting mesa at layer 158 issubject to an ion implant 171 of p-type ions, which contacts the p-typeQW inversion channel(s). Also an insulating implant 173 (utilizing, forexample, oxygen as shown) is performed under the n-type ion implant 170for the drain terminal electrode side of the device to reduce thecapacitance for high speed operation. An insulating implant (not shown)may also be performed under the p+-type implant 171 for this samepurpose. Next the device is subjected to a rapid thermal anneal (RTA) ofthe order of 900° C. or greater to activate all implants. Then thedevice is isolated from other devices by an etch down to thesemi-insulating substrate 149, which includes an etch through the mirrorpairs 151/152 of AlAs/GaAs. At this point, the device is oxidized in asteam ambient to create layers (not shown) which form the top DBRmirror. During this oxidation step, the exposed sidewalls of the etchedAlGaAs layers are passivated by the formation of very thin layers ofoxide. The final step in the fabrication is the deposition (preferablyvia lift off) of metal contacts. These contacts come in two forms. Oneis the metal layer 176 (preferably comprising an n-type Au alloy metalsuch as AuGe/Ni/Au) deposited on the N+ type implants 170 to form thesource terminal electrode 42 and drain terminal electrode 44 of thedevice. The other is the metal layer 178 (preferably comprising anp-type Au metal alloy such as AuZn/Cr/Au) deposited on the P+ typeimplant 171 to form the collector terminal electrode 48 of the device.

FIGS. 3C and 3D illustrate the operational characteristics of then-channel HFET devices of FIGS. 3A and 3B. The n-channel HFET device isan enhancement-mode device with a positive voltage level of V_(GS)turning-on the device. Under normal operation, the drain terminalelectrode 44 is forward biased with respect to the source terminalelectrode 42 by a positive voltage level V_(DS), and the gate terminalelectrode 46 is forward biased with respect to the source terminalelectrode 42 by a positive voltage level V_(GS) as shown in FIGS. 3C and3D. For small values of V_(DS), the device operates in the triode regionwhere the current I_(D) varies in a quasi-linear manner with respect toV_(DS) as shown in FIG. 3D. For larger values of V_(DS), the deviceoperates in the constant current region where the current I_(D) issubstantially constant with respect to V_(DS) as shown in FIG. 3D. Thecollector terminal electrode 48 of the device (not shown) is preferablyconnected as a back gate similar to the substrate contact in asilicon-based MOSFET transistor, and provides for bias control of theregion under the N-type QW inversion channel(s) of structure 24. Morespecifically, a negative bias on the collector terminal electrode 48with respect to the source terminal electrode 42 causes an increase inthe turn-on voltage of the n-channel HFET device. This bias voltage alsoincreases the diode depletion region width, thereby decreasing theparasitic node capacitance.

FIG. 4A illustrates an exemplary p-channel HFET device realized from themultilayer sandwich of FIG. 1A. As shown, a source terminal electrode 50and a drain terminal electrode 52 are electrically coupled to the p-typeQW structure 20 to form a channel region therebetween. One or more gateterminal electrodes (two shown as 54A, 54B) are electrically coupled tothe ohmic contact layer 14 below the p-type QW inversion channel.Preferably, an N+ implant region 49 is formed at the top of the deviceand is electrically coupled to the n-type QW structure 24, and acollector terminal electrode 56 is formed on the N+ implant region 49above the p-type QW inversion channel. When forming the p-channel HFETdevice via etching and metallization, etch stop layer 16 is used as anetch stop in order to form contacts that are electrically coupled to theohmic contact layer 14 (such contacts are subsequently metallized toform the gate terminal electrodes 54A, 54B of the p-channel HFETdevice). In this configuration, the collector terminal electrode 56 isconnected as a back gate similar to the substrate contact in asilicon-based MOSFET transistor.

FIG. 4B illustrates an exemplary p-channel HFET device realized from themultilayer sandwich of FIGS. 1B and 1C. As shown, ohmic contact layers165 a and 165 b are removed via etching, and an N+ ion implant 49 isformed (preferably to a depth of layer 162 as shown) that iselectrically coupled to the n-type QW inversion channel(s). A metallayer 174 (preferably comprising tungsten) is deposited on the N+ ionimplant 49 to form the collector terminal electrode 56 of the device.The structure outside the collector terminal electrode 56 is etchedpreferably down to layer 158, and the resulting mesas at layer 158 arethen subject to an ion implant 171 of p-type ions, which contacts thep-type QW inversion channel(s). Connection to the gate terminal (N+layer 153) of the device is made by etching with a chlorine-based gasmixture that includes fluorine. This etch is performed down to the AlAsetch stop layer 166 a. This layer 166 a is then easily dissolved inde-ionized (DI) water or wet buffered hydrofluoric acid (BHF) to formresulting mesas in the N+ layer 153. Next the device is subjected to arapid thermal anneal (RTA) of the order of 900° C. or greater toactivate all implants. Then the device is isolated from other devices byan etch down to the semi-insulating substrate 149, which includes anetch through the mirror pairs 151/152 of AlAs/GaAs. At this point, thedevice is oxidized in a steam ambient to create layers (not shown) whichform the top DBR mirror. During this oxidation step, the exposedsidewalls of the etched AlGaAs layers are passivated by the formation ofvery thin layers of oxide. The final step in the fabrication is thedeposition (preferably via lift off) of metal contacts. These contactscome in two forms. One is the metal layer 178 (preferably comprising anp-type Au metal alloy such as AuZn/Cr/Au) deposited on the P+ typeimplant 171 to form the source terminal electrode 50 and the drainterminal electrode 52 of the device. The other is metal layer 181(preferably comprising an n-type Au alloy metal such as AuGe/Ni/Au)deposited on the mesas at the N+ layer 153 to formed the gate terminalelectrodes 54A, 54B of the device.

FIG. 4C illustrates another exemplary p-channel HFET device realizedfrom the multilayer sandwich of FIGS. 1B and 1C. This device isfabricated in same manner as described above with respect to FIG. 4B,except that layer 164 (in addition to ohmic contact layers 165 a and 165b) is removed via etching before performing the N+ ion implant 49 intolayers 163 c, 163 b, 163 a (collectively 163 as shown).

FIGS. 4D and 4E illustrate the operational characteristics of thep-channel HFET devices of FIGS. 4A, 4B and 4C. The p-channel HFET deviceis an enhancement-mode device with a negative voltage level of V_(GS)turning-on the device. Under normal operation, the source terminalelectrode 50 is forward biased with respect to the drain terminalelectrode 52 by a positive voltage level V_(SD), and the gate terminalelectrode 54 is reverse biased with respect to the source terminalelectrode 50 by a negative voltage level V_(GS) as shown in FIGS. 4D and4E. For small values of V_(SD), the device operates in the triode regionwhere the current I_(D) varies in a quasi-linear manner with respect toV_(SD) as shown in FIG. 4E. For larger values of V_(SD), the deviceoperates in the constant current region where the current I_(D) issubstantially constant with respect to V_(SD) as shown in FIG. 4E. Thecollector terminal electrode 56 of the device (not shown) is preferablyconnected as a back gate similar to the substrate contact in asilicon-based MOSFET transistor, and provides for bias control of theregion above the p-type QW inversion channel(s) of structure 20. Morespecifically, a positive bias on the collector terminal electrode 56with respect to the source terminal electrode 50 causes a decrease inthe turn-on voltage of the p-channel HFET device. This bias voltage alsoincreases the diode depletion region width, thereby decreasing theparasitic node capacitance.

FIG. 5A illustrates an exemplary p-type quantum-well-base bipolartransistor device realized from the multilayer sandwich of FIG. 1A. Asshown, at least one base terminal electrode (two shown as 58A, 58B) areelectrically coupled to the p-type QW structure 20. One or more emitterterminal electrodes (two shown as 60A, 60B) are electrically coupled tothe ohmic contact layer 14 below the p-type QW structure 20. Preferably,an N+ implant region 49 is formed at the top of the device and iselectrically coupled to the n-type QW structure 24, and a collectorterminal electrode 62 is formed on the N+ implant region 49 above thep-type QW structure 20. When forming the p-type quantum-well-basebipolar transistor device via etching and metallization, etch stop layer16 is used as an etch stop in order to form contacts that areelectrically coupled to the ohmic contact layer 14 (such contacts aresubsequently metallized to form the emitter terminal electrodes 60A, 60Bof the p-type quantum-well-base bipolar transistor device).

FIG. 5B illustrates an exemplary p-type quantum-well-base bipolartransistor device realized from the multilayer sandwich of FIGS. 1B and1C. As shown, ohmic contact layers 165 a and 165 b are removed viaetching, and an N+ ion implant 49 is formed (preferably to a depth oflayer 163 c as shown) that is electrically coupled to the n-type QWstructure 24. A metal layer 174 (preferably comprising tungsten) isdeposited on the N+ ion implant 49 to form a first part of the collectorterminal electrode 62 of the device. On one side of the device, thestructure outside the first part of the collector terminal electrode 62is etched down to the etch stop layer 168 b to form a mesa at layer 163c. During this operation, a chlorine-based gas mixture that includesfluorine is used as an etchant to etch down to the etch-stop layer 168b. This etch stop layer 168 b is then easily dissolved in de-ionized(DI) water or wet buffered hydrofluoric acid (BHF) to form mesas at theundoped GaAs layer 168 a. The resulting mesa at the undoped GaAs layer168 a is then subject to N+ ion implant 170, which is electricallycoupled to the n-type QW structure 24. On the other side of the device,the resulting structure is etched preferably down to layer 158 to form amesa at layer 158, and the resulting mesa at layer 158 is subject to anion implant 171 of p-type ions, which contacts the p-type QW structure20. Connection to the emitter terminal (N+ layer 153) of the device ismade by etching with a chlorine-based gas mixture that includesfluorine. This etch is performed down to the AlAs etch stop layer 166 a.This layer 166 a is then easily dissolved in de-ionized (DI) water orwet buffered hydrofluoric acid (BHF) to form resulting mesas in the N+layer 153. Next the device is subjected to a rapid thermal anneal (RTA)of the order of 900° C. or greater to activate all implants. Then thedevice is isolated from other devices by an etch down to thesemi-insulating substrate 149, which includes an etch through the mirrorpairs 151/152 of AlAs/GaAs. At this point, the device is oxidized in asteam ambient to create layers (not shown) which form the top DBRmirror. During this oxidation step, the exposed sidewalls of the etchedAlGaAs layers are passivated by the formation of very thin layers ofoxide. The final step in the fabrication is the deposition (preferablyvia lift off) of metal contacts. These contacts come in three forms. Oneis the metal layer 176 (preferably comprising an n-type Au alloy metalsuch as AuGe/Ni/Au) deposited on the N+ type implant 170 to form thesecond part of the collector terminal electrode 62 of the device (whichis electrically connected to the first part by additional metal layersthat are not shown). The second is the metal layer 178 (preferablycomprising an p-type Au metal alloy such as AuZn/Cr/Au) deposited on theP+ type implant 171 to form the base terminal electrode 58 of thedevice. The third is metal layer 181 (preferably comprising an n-type Aualloy metal such as AuGe/Ni/Au) deposited on the mesas at the N+ layer153 to form the emitter terminal electrodes 60A, 60B of the device.

FIG. 5C illustrates another exemplary p-type quantum-well-base bipolartransistor realized from the multilayer sandwich of FIGS. 1B and 1C.This device is fabricated in same manner as described above with respectto FIG. 5B, except that layer 164 (in addition to ohmic contact layers165 a and 165 b) is removed via etching before performing the N+ ionimplant 49 into layers 163 c, 163 b, 163 a (collectively 163 as shown).In addition, the steps in forming the N+ ion implant 170 (and the secondpart of the collector terminal electrode 62 thereon) are omitted.Moreover, base terminal electrodes 58A, 58B are formed on both sides ofthe p-type QW structure 20 as shown.

FIGS. 5D and 5E illustrate the operational characteristics of the p-typequantum-well-base bipolar transistor devices of FIGS. 5A, 5B and 5C.Under normal operation, the base terminal electrode 58 is forward biasedwith respect to the emitter terminal electrode 60 by a voltage levelV_(BE), and the collector terminal electrode 62 is forward biased withrespect to the emitter terminal electrode 60 by a voltage level V_(CE)as shown in FIG. 5D. For small values of V_(CE), the device operates inthe saturation region where the current I_(C) varies in a quasi-linearmanner with respect to V_(CE) as shown in FIG. 5E. For larger values ofV_(CE), the device operates in the constant current region where thecurrent I_(C) is substantially constant with respect to V_(CE) as shownin FIG. 5E.

FIG. 6A illustrates an exemplary n-type quantum-well-base bipolartransistor device realized from the multilayer sandwich of FIG. 1A. Asshown, at least one base terminal electrode (two shown as 64A, 64B) areelectrically coupled to the n-type QW structure 24. One or morecollector terminal electrodes (two shown as 68A, 68B) are electricallycoupled to the p-type QW structure 20. An emitter terminal electrode 66is formed on the ohmic contact layer 30. When forming the n-typequantum-well-base bipolar transistor device via etching andmetallization, etch stop layer 28 a is used as an etch stop in order toform contacts that are electrically coupled to the n-type QW structure24 (such contacts are subsequently metallized to form the base terminalelectrodes 64A, 64B).

FIG. 6B illustrates an exemplary n-type quantum-well-base bipolartransistor realized from the multilayer sandwich of FIGS. 1B and 1C. Asshown, a metal layer 174 (preferably comprising tungsten) deposited onthe ohmic contact layer 165 b forms the emitter terminal electrode 66 ofthe device. The structure outside the emitter terminal electrode 66 isetched down to the etch stop layer 168 b. During this operation, achlorine-based gas mixture that includes fluorine is used as an etchantto etch down to the etch-stop layer 168 b. This etch stop layer 168 b isthen easily dissolved in de-ionized (DI) water or wet bufferedhydrofluoric acid (BHF) to form mesas at the undoped GaAs layer 168 a.The resulting mesas at the undoped GaAs layer 168 a are then subject toN+ ion implants 170, which are electrically coupled to the n-type QWstructure 24. The resulting structure is etched preferably down to layer158, and the resulting mesas at layer 158 are then subject to an ionimplant 171 of p-type ions, which contacts the p-type QW structure 20.Next the device is subjected to a rapid thermal anneal (RTA) of theorder of 900° C. or greater to activate all implants. Then the device isisolated from other devices by an etch down to the semi-insulatingsubstrate 149, which includes an etch through the mirror pairs 151/152of AlAs/GaAs. At this point, the device is oxidized in a steam ambientto create layers (not shown) which form the top DBR mirror. During thisoxidation step, the exposed sidewalls of the etched AlGaAs layers arepassivated by the formation of very thin layers of oxide. The final stepin the fabrication is the deposition (preferably via lift off) of metalcontacts. These contacts come in two forms. One is the metal layer 176(preferably comprising an n-type Au alloy metal such as AuGe/Ni/Au)deposited on the N+ type implants 170 to form the base terminalelectrodes 64A, 64B of the device. The other is the metal layer 178(preferably comprising an p-type Au metal alloy such as AuZn/Cr/Au)deposited on the P+ type implants 171 to form the collector terminalelectrodes 68A, 68B of the device.

FIGS. 6C and 6D illustrate the operational characteristics of the n-typequantum-well-base bipolar transistor devices of FIGS. 6A and 6B. Undernormal operation, the base terminal electrode 64 is reverse biased withrespect to the emitter terminal electrode 66 by a voltage level V_(EB),and the collector terminal electrode 68 is reverse biased with respectto the emitter terminal electrode 66 by a voltage level V_(EC) as shownin FIG. 6C. For small values of V_(EC), the device operates in thesaturation region where the current I_(C) varies in a quasi-linearmanner with respect to V_(EC) as shown in FIG. 6D. For larger values ofV_(EC), the device operates in the constant current region where thecurrent I_(C) is substantially constant with respect to V_(EC) as shownin FIG. 6D.

FIG. 7 is a flow chart illustrating an exemplary method of fabricatingthe multilayer structure of FIG. 1A to integrate the variousoptoelectronic/electronic devices described herein (including theheterojunction thyristor device of FIG. 2B, the n-channel HFET device ofFIG. 3B, the p-channel HFET devices of FIGS. 4B and 4C, the p-typequantum-well-base bipolar transistor devices of FIGS. 5B and 5C, and then-type quantum-well-base bipolar transistor device of FIG. 6B) on acommon substrate. The methodology begins in block B2 by patterning andetching the structure over the active region of the each n-typequantum-well-base bipolar device and each p-channel HFET device. Theetching operation of block B2 is controlled such that it terminates atlayer 164 (or alternatively, at layer 163 c).

Then, in block B4, an implant of n-type ions is performed to form theN-type implants 175 (of each heterojunction thyristor device) inaddition to the N+-type implant 49 (of each n-type quantum-well-basebipolar device and each p-channel HFET device). Preferably, the N-typeimplants 175 are implanted into the p-type ohmic contact layers 165b/165 a to a depth near layer 162 as shown in FIG. 2B, and the N+-typeimplant 49 is implanted into the layer 164 (or layer 163 c) that isexposed by the etching operation of step B2 to a depth of layer 162 asshown in FIGS. 4B, 4C and 5C.

In block B6, a metal layer 174 (preferably comprising tungsten) isdeposited and defined to form electrodes for the various devices. Aspart of block B6, metal layer 174 is deposited on the ohmic contactlayer 165 b above the N-type implants 175 to form the anode terminalelectrodes 36A, 36B for each heterojunction thyristor device as shown inFIG. 2B. The metal layer 174 is also deposited above the N+-type implant49 to form the collector electrode 56 of each p-channel HFET device (asshown in FIGS. 4B and 4C) and the collector electrode 62 for each n-typequantum-well-base bipolar transistor device (as shown in FIGS. 5B and5C). In addition, the metal layer 174 is deposited on the ohmic contactlayer 165 b to form the gate terminal electrode 46 of each n-channelHFET device (as shown in FIG. 3B) and the emitter terminal electrode 66for each n-type quantum-well-base bipolar transistor device (as shown inFIG. 6B).

In block B8, the resultant structure of block B6 is subject to apatterning and etching operation that exposes regions of layer 168 a.During this operation, a chlorine-based gas mixture that includesfluorine is used as an etchant to etch down to the etch-stop layer 168 bas described above. An implant of n-type ions is implanted into theexposed regions of layer 168 a to form N+-type implants 170, which isused to contact to the n-type QW structure 24 for each heterojunctionthyristor device (as shown in FIG. 2B), for each n-channel HFET device(as shown in FIG. 3B), for each n-type quantum-well-base bipolartransistor device (as shown in FIG. 6B), and possibly for each p-typequantum-well-base bipolar transistor device (as shown in FIG. 5B).Advantageously, the N+-type implants 170 are self-aligned by theelectrodes formed on the mesas above the implants 170 as shown in theseFigures.

In block B10, the resultant structure of block B8 is subject to anetching operation that exposes regions preferably at or near layer 158.An implant of p-type ions is implanted into the exposed regions to formthe P+-type implants 171, which are used to contact the p-type QWstructure 20 for each heterojunction thyristor device (as shown in FIG.2B), for each n-channel HFET device (as shown in FIG. 3B), for eachp-channel HFET device (as shown in FIGS. 4B and 4C), for each p-typequantum-well-base bipolar transistor device (as shown in FIGS. 5B and5C), and for each n-type quantum-well-base bipolar transistor device (asshown in FIG. 6B). Advantageously, the P+-type implants 171 areself-aligned by the N+-type implants 170 formed on the mesas above theimplants 171.

In block B12, the resultant structure of block B10 is subject to apatterning and etching operation that exposes regions of ohmic contactlayer 153. During this operation, a chlorine-based gas mixture thatincludes fluorine is used as an etchant to etch down to the etch-stoplayer 166 a as described above. The exposed regions of layer 153 areused to form a low resistance contact to electrodes for the variousdevices, including the cathode terminal electrodes 40A, 40B of eachheterojunction thyristor device (as shown in FIG. 2B), the gate terminalelectrodes 54A, 54B for each p-channel HFET device (as shown in FIGS. 4Band 4C), and the emitter terminal electrodes 60A, 60B for each p-typequantum-well-base bipolar transistor device (as shown in FIGS. 5B and5C). In addition, the device is subjected to a rapid thermal anneal(RTA) of the order of 900° C. or greater to activate all implants. Thenthe device is isolated from other devices by an etch down to thesemi-insulating substrate 149, which includes an etch through the mirrorpairs 151/152 of AlAs/GaAs. At this point, the device is oxidized in asteam ambient to convert layers 151 to AlO, which form the bottom DBRmirror. During this oxidation step, the exposed sidewalls of the etchedAlGaAs layers are passivated by the formation of very thin layers ofoxide. In addition, the layers 179/180 are deposited to form the top DBRmirror. Preferably, the layers 179/80 comprise SiO₂ and a highrefractive index material such as GaAs, Si, or GaN.

Finally, in block B14, metal layers 176, 178 and 181 are deposited anddefined (preferably via lift off). Metal layer 176 (which preferablycomprises an n-type Au alloy metal such as AuGe/Ni/Au) is deposited onthe N+ type implants 170 to form the N-channel injector terminalelectrodes 38A, 38B of each heterojunction thyristor device (as shown inFIG. 2B), the source terminal electrode 42 and drain terminal electrode44 of each n-channel HFET device (as shown in FIG. 3B), the baseterminal electrodes 64A, 64B of each n-type quantum-well-base bipolartransistor device (as shown in FIG. 6B), and possibly a portion of thecollector terminal electrode 62 of each p-type quantum-well-base bipolartransistor device (as shown in FIG. 5B). Metal layer 178 (whichpreferably comprises a p-type Au metal alloy such as AuZn/Cr/Au) isdeposited on the P+ type implants 171 to form the p-channel injectorterminal electrodes 38C, 38D of each heterojunction thyristor device (asshown in FIG. 2B), the source terminal electrode 50 and drain terminalelectrode 52 of each p-channel HFET device (as shown in FIGS. 4B and4C), the base terminal electrodes 58A, 58B of each p-typequantum-well-base bipolar transistor device (as shown in FIGS. 5B and5C), and the collector terminal electrodes 68A, 68B of each n-typequantum-well-base bipolar transistor device (as shown in FIG. 6B). Metallayer 181 (which preferably comprises an n-type Au alloy metal such asAuGe/Ni/Au) is deposited on the mesas at the N+ layer 153 to formed thecathode terminal electrodes 40A, 40B of each heterojunction thyristordevice (as shown in FIG. 2B), the gate terminal electrodes 54A, 54B ofeach p-channel HFET device (as shown in FIGS. 4B and 4C), and theemitter terminal electrodes 60A, 60B of each p-type quantum-well-basebipolar transistor device (as shown in FIGS. 5B and 5C).

For high performance quantum-well-base bipolar transistor devices, it ispreferable that the vertical distance between QW base and theemitter/collector of the device be minimized. Such reduced verticaldimensions reduces the transit time delay of charge passingtherethrough, and thus provides for higher frequency operation.Advantageously, the vertical dimension between the n-type ohmic contactlayer 153 and the first quantum well in structure 20 in addition to thevertical dimension between the last quantum well in structure 24 and thetop electrode metal layer 174 can be made small (e.g., on the order of370-655 Å) to provide for high frequency operation.

For a high performance p-type quantum-well-base bipolar transistordevice realized from the multilayer structures described herein, it ispreferable that the effective area of the base-collector junction inaddition to the effective area of the base-emitter junction beminimized. This reduces the base-collector capacitance and thebase-emitter capacitance, and thus provides for higher frequencyoperation. Moreover, it is preferable that the resistance of the baseterminal, the resistance of the collector terminal and the resistance ofthe emitter terminal be minimized to provide for higher frequencyoperation. In the p-type quantum-well-base bipolar transistor devicesdescribed herein, the effective area of the base-collector junction iscontrolled by the dimensions of the collector electrode metal layer 174.The resistance of the collector is minimized by controlling the dopingconcentration of the collector contact (N+ implant 49).

Turning now to FIGS. 8A through 8C2, the effective area of thebase-emitter junction in addition to the base terminal resistance andemitter terminal resistance are minimized by interdigitization of theP+-type implants 171 on both sides of the collector metal layer 174.Such interdigitization is preferably accomplished as part of theoperations of blocks B10 and B12 as described above with respect to FIG.7. As shown in FIG. 8A, the P+ implants 171 are formed in selected areason both sides of the collector metal layer 174. Importantly, theseimplants 171 are deep to a point near the dielectric layer 151 as shownin FIGS. 8B1 and 8C1, which reduces the effective area of thebase-emitter junction, and eliminates much of the capacitance betweenthe base and the emitter (e.g., the capacitance is reduced to that whichexists along the sidewalls of the implants 171). Between the selectedareas of the P+ implants 171, the structure is etched down through thep-type quantum well structure 20 to expose the n+ ohmic contact layer153 for metallization/contact thereto as shown in FIGS. 8A, 8B2 and 8C2.Advantageously, the finger regions of metal layers 178/181 that are partof the base terminal electrode 58 and emitter terminal electrode 60 asshown in FIG. 8A provide very low base terminal resistance and emitterterminal resistance, respectively. In addition, because the implants are171 are self-aligned to the metal layer 174, the width of the metallayer 174 may be minimized (preferably, to sub-micron widths). All ofthese features contribute to higher frequency operation of the device.

FIGS. 8B1 and 8B2 illustrate an exemplary p-type quantum-well-basetransistor that is realized by interdigitization of the P+-type implants171 on both sides of the collector metal layer 174 as shown in FIG. 8A.FIG. 8B1 illustrates the cross-section A-A of the device as shown inFIG. 8A, and FIG. 8B2 illustrates the cross-section B-B of the device asshown in FIG. 8A. Both cross-sections are similar to that describedabove with respect to the p-type quantum-well-base transistor device ofFIG. 5C. Note that in FIG. 8C1, the P+ implants 171 are deep to a pointnear the dielectric layer 151, which reduces the effective area of thebase-emitter junction, and eliminates much of the capacitance betweenthe base and the emitter (e.g., the capacitance is reduced to that whichexists along the sidewalls of the implants 171).

FIGS. 8C1 and 8C2 illustrate another exemplary p-type quantum-well-basetransistor that is realized by interdigitization of the P+-type implants171 on both sides of the collector metal layer 174 as shown in FIG. 8A.FIG. 8C1 illustrates the cross-section A-A of the device as shown inFIG. 8A, and FIG. 8C2 illustrates the cross-section B-B of the device asshown in FIG. 8A. Both cross-sections are similar to that describedabove with respect to the device of FIG. 5C; however, layer 164 is notetched away prior to the collector n-type implant 49. Thus, thecollector terminal electrode 62 is subsequently formed thereon as shown.Note that in FIG. 8C1, the P+ implants 171 are deep to a point near thedielectric layer 151, which reduces the effective area of thebase-emitter junction, and eliminates much of the capacitance betweenthe base and the emitter (e.g., the capacitance is reduced to that whichexists along the sidewalls of the implants 171).

There are many advantages gained by the semiconductor device structuredescribed herein including: the FET capacitance and position of the gatevoltage control are de-coupled from the doping used to achieve low gatecontact resistance, the incidence of gate to source short circuits isgreatly reduced, the effective (electrical) thickness of the gatedielectric can be made exceedingly thin, the layers can be accuratelyetched away to achieve low contact resistance, the threshold can be moreeasily adjusted by implant to obtain depletion devices, andmanufacturability is much improved. Moreover, a broad array ofoptoelectronic devices can be integrated to form a monolithicoptoelectronic integrated circuit suitable for many diverseapplications. Such devices include optoelectronic thyristor. Thethyristor has unique properties of sensitive detection in its OFF stateand laser emission in its ON state. The thyristor structure may be usedas a digital modulator, a transceiver, an amplifier and a directionalcoupler. These devices may be realized as either waveguide or verticalcavity devices. The vertical cavity construction enables resonant cavityoperation of all device modes. In addition to the multipleoptoelectronic devices, a wide array of transistor devices (includingcomplementary HFET devices and complementary quantum-well-base bipolartransistors) are implementable.

There have been described and illustrated herein several embodiments ofa semiconductor device employing at least one modulation doped quantumwell structure and one or more etch stop layers for accurate contactformation and a method of fabricating such semiconductor devices toimplement thyristors, transistors, optical emitters, optical detectors,optical modulators, optical amplifiers and other opto-electronicdevices. While particular embodiments of the invention have beendescribed, it is not intended that the invention be limited thereto, asit is intended that the invention be as broad in scope as the art willallow and that the specification be read likewise. Thus, whileparticular layers have been described with particular thicknesses andwith particular types and strengths of dopings, it will be appreciatedthat certain transition layers could be removed and/or additional layersand/or sublayers could be utilized, and further that the layers couldhave different thicknesses and be differently doped. Also, whileparticular layers have been described with reference to their percentagecontent of certain constituents, it will be appreciated that the layerscould utilize the same constituents with different percentages, or otherconstituents. Additionally, while particular formation and metallizationtechniques have been described, it will be appreciated that thedescribed structures can be formed in other manners, and other metalsused to form terminals. Further, while particular arrangements ofbipolar and FET transistors, optical emitters, detectors, modulators,amplifiers, etc. formed from the described semiconductor structure, andcircuits utilizing those components have been described, it will beappreciated that other devices and circuits can be made from theprovided structure and components. It will therefore be appreciated bythose skilled in the art that yet other modifications could be made tothe provided invention without deviating therefrom.

1. A method of fabricating a semiconductor device comprising the stepsof: providing a series of layers formed on a substrate, said layersincluding a first plurality of layers comprising n-type dopant material,a second plurality of layers that form a p-type modulation doped quantumwell structure, and a third plurality of layers including at least onelayer comprising n-type dopant material, wherein said first plurality oflayers includes an n-type ohmic contact layer and a first etch stoplayer formed above said n-type ohmic contact layer for contacting saidn-type ohmic contact layer; performing an etching operation thatautomatically stops at said first etch stop layer; removing remainingportions of said first etch stop layer to expose first areas of saidn-type ohmic contact layer; depositing a first metal layer on said firstareas of said n-type ohmic contact layer to form a gate terminalelectrode of a first-type transistor device; and depositing a secondmetal layer that is electrically coupled to said p-type modulation dopedquantum well structure to form a source terminal electrode and a drainterminal electrode of said first-type transistor device.
 2. A method offabricating a semiconductor device according to claim 1, furthercomprising the steps of: performing a first implant of p-type ions toform at least one p-type implant region that is electrically coupled tosaid p-type modulation doped quantum well structure; and depositing saidsecond metal layer on said at least one p-type implant region.
 3. Amethod of fabricating a semiconductor device according to claim 1,further comprising the step of: depositing a third metal layer that isoperably coupled to said third plurality of layers to form a collectorterminal electrode of said first-type transistor device.
 4. A method offabricating a semiconductor device according to claim 3, furthercomprising the steps of: performing a second implant of n-type ions toform an n-type implant region that is electrically coupled to said thirdplurality of layers, and depositing said third metal layer on saidn-type implant region.
 5. A method of fabricating a semiconductor deviceaccording to claim 3, wherein: said third metal layer is deposited priorto said first and second metal layers.
 6. A method of fabricating asemiconductor device according to claim 1, further comprising the stepsof: providing said series of layers with a fourth plurality of layerscomprising p-type dopant material and a p-type ohmic contact layer,wherein said third plurality of layers forms an n-type modulation dopedquantum well structure; and depositing a second metal layer that iselectrically coupled to said n-type modulation doped quantum wellstructure to form a source terminal electrode and a drain terminalelectrode of a second-type transistor device; and depositing a thirdmetal layer that is electrically coupled to said p-type ohmic contactlayer to form a gate terminal electrode of said second-type transistordevice.
 7. A method of fabricating a semiconductor device according toclaim 6, further comprising the steps of: performing a first implant ofn-type ions to form at least one n-type implant region that iselectrically coupled to said n-type modulation doped quantum wellstructure; and depositing said second metal layer on said at least onen-type implant region.
 8. A method of fabricating a semiconductor deviceaccording to claim 6, further comprising the step of: depositing afourth metal layer that is operably coupled to said p-type modulationdoped quantum well structure to form a collector terminal electrode ofsaid second-type transistor device.
 9. A method of fabricating asemiconductor device according to claim 8, further comprising the stepsof: performing a second implant of p-type ions to form at least onep-type implant region that is electrically coupled to said p-typemodulation doped quantum well structure, and depositing said fourthmetal layer on said at least one p-type implant region.
 10. A method offabricating a semiconductor device according to claim 9, wherein: saidthird metal layer is deposited prior to said second and fourth metallayers.
 11. A method of fabricating a semiconductor device comprisingthe steps of: providing a series of layers formed on a substrate, saidlayers including a first plurality of layers including at least onelayer comprising p-type dopant material, a second plurality of layersthat form an n-type modulation doped quantum well structure, and a thirdplurality of layers including at least one layer comprising p-typedopant material, wherein said third plurality of layers includes ap-type ohmic contact layer and a first etch stop layer formed above saidn-type modulation doped quantum well structure for contacting saidn-type modulation doped quantum well structure; depositing a first metallayer on said p-type ohmic contact layer to form a gate terminalelectrode of a first-type transistor device; performing an etchingoperation that automatically stops at said first etch stop layer;removing remaining portions of said first etch stop layer to exposefirst areas of a layer thereunder; and depositing a second metal layeron said first areas to form a source terminal electrode and a drainterminal electrode of said first-type transistor device, wherein saidsource terminal electrode and said drain terminal electrode areelectrically coupled to said n-type modulation doped quantum wellstructure.
 12. A method of fabricating a semiconductor device accordingto claim 11, wherein: said first etch stop layer permits currenttunneling.
 13. A method of fabricating a semiconductor device accordingto claim 11, further comprising the steps of: performing a first implantof n-type ions in said first areas to form at least one n-type implantregion that is electrically coupled to said n-type modulation dopedquantum well structure; and depositing said second metal layer on saidat least one n-type implant region.
 14. A method of fabricating asemiconductor device according to claim 11, wherein: said firstplurality of layers forms a p-type modulation doped quantum wellstructure.
 15. A method of fabricating a semiconductor device accordingto claim 14, wherein: said series of layers further comprises a firstplurality of undoped spacer layers disposed between said first pluralityof layers and said second plurality of layers, and a second plurality ofundoped spacer layers disposed between said second plurality of layersand said third plurality of layers, wherein said second plurality ofundoped spacer layers include a thin capping layer.
 16. A method offabricating a semiconductor device according to claim 15, furthercomprising the steps of: performing an etching operation that exposessecond areas between said n-type modulation doped structure and saidp-type modulation doped structure; depositing a third metal layer onsaid second areas to form a collector electrode of said first-typetransistor device, wherein said collector electrode is electricallycoupled to said p-type modulation doped quantum well structure.
 17. Amethod of fabricating a semiconductor device according to claim 16,further comprising the steps of: performing a second implant of p-typeions in said second areas to form at least one p-type implant regionthat is electrically coupled to said p-type modulation doped quantumwell structure; and depositing said third metal layer on said at leastone p-type implant region.